The invention relates to a method according to the preambles of independent claims 1 and 5 for defining an instantaneous value of a current of a pulse-controlled inductive load when the impedance of the load is known, the method comprising the steps of
measuring the output voltage of a pulsed voltage source, and PA1 measuring the output current of the pulsed voltage source. PA1 low-pass filtering the measured output current of the pulsed voltage source to produce a fundamental wave current, PA1 defining a load current estimate by computation on the basis of the measured output voltage of the pulsed voltage source and the impedance of the load, PA1 high-pass filtering the load current estimate, and PA1 defining the instantaneous value of the load current by adding the high-pass-filtered load current estimate to the fundamental wave current. PA1 adding a correction term to the output voltage of the pulsed voltage source to produce an estimation voltage; PA1 defining a load current estimate by computation on the basis of the estimation voltage and the impedance of the load, whereby the load current estimate provides the instantaneous value of the load current; PA1 low-pass filtering the load current estimate; PA1 low-pass filtering the measured output current of the pulsed voltage source to produce a fundamental wave current; PA1 comparing the fundamental wave current with the low-pass-filtered load current estimate to produce an error parameter proportional to the difference of the currents; and PA1 multiplying the error parameter by a coefficient to produce the correction term.
The current or magnetic flux of the inductive load is normally controlled by changing the voltage affecting over the load, the voltage typically consisting of single or multi-level voltage pulses generated by semiconductor switches. The simplest solution is that the switches operate at a fixed switching frequency, whereby the pulse width of the voltage pulses determines the average voltage level in the load. An example for such a pulsed voltage source is a pulse-width-modulated (PWM) voltage source. The current of the load is here quite well controlled dynamically, if the switching frequency is sufficiently high.
On account of the switching losses of the semiconductor switches, the switching frequency is kept as low as possible, particularly in connection with high-power apparatus, such as frequency converters that control squirrel-cage induction motors. Accurate dynamic control of the current of the load can here be achieved only if the switching of the switches is based directly on the instantaneous values of the current of the load. The pulsed voltage sources based on the use of instantaneous values of the current of the load include, for example, voltage sources that are based on the Direct Torque Control (DTC) and tolerance band control of the current. The methods work well when the load is close to the voltage source that generates voltage pulses, and when there are no capacitive components between the load and the pulsed voltage source, whereby the current of the load can be measured in undisturbed conditions.
In practice, however, the load is often situated at a relatively long distance from the voltage source supplying it. The instantaneous current of the pulsed voltage source then differs from the instantaneous current of the load on account of the currents passing through the stray capacitances in the supply cable. The reason for this is that the transfer impedance of the cable is usually much lower than the impedance of the inductive load. Because the impedances are not equal, each voltage pulse supplied generates voltage oscillation at that end of the supply cable which is close to the load, and current oscillation at that end of the supply cable which is close to the voltage source.
The modulation basis used is usually the current measured at the end of the voltage source; above a certain length of cable the measurements are so inaccurate due to the current oscillation that the instantaneous value of the load current can no longer be controlled. When long cables are used, either the dynamics of the control has to be compromised or the current of the load has to be measured separately at the load, whereby expensive separate cabling has to be installed to enable the transfer of the measuring signal. Apart from the long cables, capacitive components between the load and the voltage source also inhibit the above kind of modulation that is based on the current measured at the end of the voltage source. Such a capacitive component can be, for example, an LC low pass filter.